Recent studies of craniofacial muscle development in chick embryos unexpectedly revealed extensive degeneration of primary myotubes midway through the embryonic period. Muscle cell death (apoptosis) is restricted to developing head and neck muscles in these embryos, suggesting that apoptosis has a role in establishing appropriate functions of craniofacial muscles. The cause(s) and significance of programmed cell death of developing muscle cells are not understood. The research proposed here will define the extent of programmed cell death in selected head and neck muscles, and will investigate possible links between fiber type and muscle cell survival. Pro- graced cell death of motor neurons is well-studied, but the impact of neuronal degeneration on primary myotubes in vivo is not known. This will be analyzed by reducing the extent of motor neuron cell death, and examining the muscles. Finally, cell death may be programmed in specific myogenic lineages. Lineage-dependent apoptosis will be assessed by transplanting non-apoptotic lineages in the place of apoptotic muscle progenitors. These experiments will elucidate the cause(s) and role of apoptosis in developing craniofacial muscles.